Structure and dynamics of hexagonal cells in H2/CO2 flames

Abstract

Transition from fossil fuels to sustainably produced hydrogen remains an important goal to achieve current climate targets. Hydrogen from gasification and steam reforming is produced alongside CO2. Instead of separating the CO2, it can be kept in the product gas, leading to possible reductions in NOx and improved safety aspects. However, the addition of CO2 makes hydrogen flames more prone to cellular instabilities, which can lead to problems when designing burners for hydrogen-based flames. To understand the formation and dynamics of these instabilities, fully resolved 3D simulations of H2/CO2 cellular flames on a heat flux burner are performed. The numerical configuration follows an experimental setup that observed cellular and band-like structures stabilizing on the burner plate. With the simulations employing finite rate chemistry and detailed diffusion models, hexagonal flame structures are identified and characterized. The simulations show good qualitative agreement of the flame structure with the measurements. The influence of mass flow rate, burner plate temperature and equivalence ratio on the flame structure is investigated. A transition from band-like to hexagonal structures is described in terms of cusp formation, which become the corner points of the hexagonal cells, and the stabilization mechanism is explained via the high local flame stretch above holes in the burner plate. A Markstein number-based correlation for the cell size with respect to the burner plate temperature is proposed and the simulation data is provided as a public database for further analysis.